Core/Movement: Prevent immediate movement generators from fully removing other movement

Author: Shauren

src/server/game/Movement/MotionMaster.cpp

@@ -1169,7 +1169,9 @@ void MotionMaster::DirectAdd(MovementGenerator* movement, MovementSlot slot/* =
             auto where = _generators.lower_bound(movement);
             if (!_generators.empty())
             {
-                bool replacesExisting = where != _generators.end() && !_generators.key_comp()(movement, *where);
+                bool replacesExisting = !movement->HasFlag(MOVEMENTGENERATOR_FLAG_IMMEDIATE)
+                    && where != _generators.end()
+                    && !_generators.key_comp()(movement, *where);
                 auto top = _generators.begin();
                 if (replacesExisting)
                     Remove(where, where == top, false);